1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles.
2. Description of the Related Art
Exhaust gas, which is discharged from vehicles or automobiles such as gasoline-fueled automobiles and diesel powered automobiles, contains nitrogen oxides (NOx) such as nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2), as well as carbon monoxide (CO), hydrocarbon (CH), hydrogen (H.sub.2), oxygen (O.sub.2) and so on. In such exhaust gas, about 80% of the entire NOx is occupied by NO, and about 95% of the entire NOx is occupied by NO and NO.sub.2.
The three way catalyst, which is used to clean HC, CO, and NOx contained in the exhaust gas, exhibits its maximum cleaning efficiency in the vicinity of the theoretical air fuel ratio (A/F=14.6). If A/F is controlled to be not less than 16, the amount of produced NOx is decreased. However, the cleaning efficiency of the catalyst is lowered, and consequently the amount of discharged NOx is apt to increase.
Recently, in order to effectively utilize fossil fuel and avoid global warming, the market demand increases, for example, in that the discharge amount of CO.sub.2 should be suppressed. In order to respond to such a demand, it becomes more necessary to improve the fuel efficiency. In response to such a demand, for example, the lean burn engine and the catalyst for cleaning NOx are being researched. Especially, the need for a NOx sensor increases.
A conventional NOx analyzer has been hitherto known in order to detect NOx as described above. The conventional NOx analyzer is operated to measure a characteristic inherent in NOx, based on the use of chemical luminous analysis. However, the conventional NOx analyzer is inconvenient in that the instrument itself is extremely large and expensive. The conventional NOx analyzer requires frequent maintenance because optical parts are used to detect NOx. Further, when the conventional NOx analyzer is used, any sampling operation should be performed for measurement of NOx, wherein it is impossible to directly insert a detecting element itself into a fluid. Therefore, the conventional NOx analyzer is not suitable for analyzing transient phenomena such as those occur in the exhaust gas discharged from an automobile, in which the condition frequently varies.
In order to dissolve the inconveniences as described above, there has been suggested and practically used a sensor for measuring a desired gas component in exhaust gas by using a substrate composed of an oxygen ion-conductive solid electrolyte.
FIG. 10 shows a cross-sectional arrangement of a gas sensor 10 disclosed in Japanese Laid-Open Patent Publication No. 8-271476.
The gas sensor 10 is operated as follows. That is, a measurement gas is introduced into a first hollow space 14 via a first diffusion rate-determining section 12. A first oxygen pumping means 22, which comprises an inner pumping electrode 16, a solid electrolyte 18, and an outer pumping electrode 20, is used to pump in or pump out oxygen contained in the measurement gas, into or from the first hollow space 14 to an extent that the measurement gas is not decomposed.
Subsequently, the measurement gas is introduced into a second hollow space 26 via a second diffusion rate-determining section 24. A second oxygen pumping means 36, which comprises a measurement gas-decomposing electrode 28 disposed in the second hollow space 26, a solid electrolyte 30, and a reference electrode 34 disposed in a reference gas-introducing space 32, is used to pump out oxygen produced by decomposition and electrolysis caused by the applied voltage or the catalytic action effected by the measurement gas-decomposing electrode 28.
The value of the current, which is required to pump out oxygen by the second oxygen pumping means 36, is measured to measure the predetermined gas component contained in the measurement gas, on the basis of the current value.
Those to which the gas sensor 10 is applicable include, for example, NOx sensors, H.sub.2 O sensors and CO.sub.2 sensors for measuring NOx, H.sub.2 O, and CO.sub.2 in which the predetermined gas component has bound oxygen.
In the case of the use as a NOx sensor, NOx is catalytically decomposed by using, for example, Rh or Pt for the measurement gas-decomposing electrode 28. The oxygen produced during the decomposition can be detected as a pumping current, or it can be detected as a change in voltage of an oxygen concentration cell.
As shown in FIG. 11, another gas sensor 10A has been suggested (see, for example, Japanese Laid-Open Patent Publication No. 9-113484), in which the oxygen dependency of the gas sensor 10 described above is improved when the gas sensor 10 is used as a NOx sensor.
The gas sensor 10A comprises an auxiliary pumping electrode 38 disposed at a second hollow space 26. A third oxygen pumping means, i.e., an auxiliary pumping means 40 is constructed by the auxiliary pumping electrode 38, solid electrolytes (including 18 and 30), and a reference electrode 34. The oxygen, which diffuses and enters from a first hollow space 14 in a minute amount, is pumped out again by using the auxiliary pumping means 40. Accordingly, it is possible to greatly improve the measurement accuracy (especially the dependency on oxygen concentration).
However, the gas sensors 10, 10A are in the following actual state of affairs. That is, even when the oxygen concentration in the measurement gas is controlled to be, for example, not more than 1 ppm in the previous stage of NOx measurement by using the first oxygen pumping means 22, or by using the first oxygen pumping means 22 and the auxiliary pumping means 40, the pumping current value at NOx=0 (hereinafter referred to as "offset value") is a value corresponding to 100 ppm which is much higher than a value corresponding to 1 ppm.
If the offset value is always constant in all environments in which the gas sensors 10, 10A are used, no problem arises. However, it is feared that a large measurement error may be caused, because the offset value varies depending on the change in temperature of exhaust gas.
In order to decrease the offset value, it is conceived that strict control is performed by using the first oxygen pumping means 22, and strict control is performed by using the auxiliary pumping means 40, for example, so that the oxygen concentration to be controlled is further decreased. However, if such strict control is performed, a problem arises in that NOx is decomposed by the pumping process effected thereby.
FIG. 12 shows a situation of the problem described above. When the oxygen concentration in the first hollow space 14 of the gas sensor 10 shown in FIG. 10 is controlled to be 10.sup.-7 atm (about 300 mV as a voltage detected by an oxygen concentration detector), the offset value is 1.0 .mu.A.
Theoretically and essentially, the offset value should be a value corresponding to a residual oxygen concentration in the first hollow space 14, i.e., 0.1 ppm (or corresponding to 0.2 ppm after conversion into a value of NO). However, the offset value is actually 1 .mu.A (or a value of NO corresponding to 200 ppm obtained by conversion). The NO sensitivity is calculated in accordance with 5 .mu.A/1000 ppm.
Therefore, if the offset value slightly changes depending on, for example, the temperature of the sensor element, and it is changed in a degree of 10%, then the resultant change corresponds to 20 ppm, which causes a serious problem when NOx is measured at a low concentration in a degree of several hundreds of ppm.
FIG. 13 shows such a situation. For example, when the temperature of the measurement gas (gas temperature) is changed by about 150.degree. C. from 650.degree. C. to 800.degree. C., the offset value is changed in an amount of about 1.5 .mu.A, i.e., 300 ppm. This causes a serious problem when NOx is measured at a low concentration of several hundreds of ppm.
In order to solve this problem, a method is conceived, in which the oxygen concentration in the first hollow space 14 is lowered. However, even when the oxygen concentration in the first hollow space 14 is lowered up to 10.sup.-10 atm, the offset value is still 1 .mu.A. When the oxygen concentration is lowered up to 10.sup.-12 atm, the offset value is finally 0.1 .mu.A (corresponding to 20 ppm). Under this condition, even when the offset value is changed by 2% due to the temperature change or the like, the amount of change is suppressed to be about 4 ppm at most, which is at a sufficient level to make measurement for several hundreds of ppm.
However, when the oxygen concentration in the first hollow space 14 is lowered to be too low, the reaction with NOx occurs in the first hollow space 14 before combustion of inflammable gas components such as HC and CO contained in exhaust gas. A new problem arises in that the decrease in sensitivity takes place.